This invention relates to an induction heating apparatus.
Two types of induction heating apparatus are known. One type is of a series inverter control system in which a heating coil is connected in series with a capacitor and the other type is of a parallel inverter control system in which a heating coil is connected in parallel with a capacitor. In either system a sinnsoidal current of high frequency is continuously or intermittently supplied to the heating coil to heat a load, such as a pan, which is located near the heating coil. Where an electric power control is effected by the above-mentioned inverter control system, the load power is controlled by phase-controlling or rectification-controlling a power source voltage using, for example, a thyristor.
Since in such conventional induction heating apparatus the dissipation power of the load is varied in proportion to substantially a square of a power source voltage, the dissipation power tends to be easily varied with respect to the variation of a power source and it is necessary to use a highly stable power source. In the conventional induction heating apparatus an electromagnetic energy stored in the heating coil is not fed back to the power source, resulting in poor electric power efficiency. In the no load state a surge voltage more than several times as large as a voltage occurring in the load state is developed and there is a chance that a thyristor for control will be destroyed by the surge voltage. Since the load electric power is controlled by an AC phase control or rectification control the apparent electric power is increased, lowering the electric power efficiency. Furthermore, a critical rate (di/dt) of rise of the ON-state current is increased and in consequence there is a tendency for radio frequency interference (RFI) to occur.